VSE: Turning split screen on and off when using multicam
I think I understand what you want, but I’m not sure what multicam is. What I would expect you to do is to have two transform strips, each attached to one of two identical video strips (identical, that is, except for one coming from a different camera). The transform strip on a higher channel should be set with a compositing setting (in strip properties) that allows the other transform strip to show through in its appropriate location. Probably “alpha over.”
When you use a transform strip, the transform strip itself is like a new version of the old video strip to the sequencer, and should be treated as such in how you composite it. I think you can actually set the transform opaqueness to half (with the right compositing setting) and see the original video underneath.